Do wild titi monkeys show empathy?
We observed a putative case of empathy among wild black-fronted titi monkeys ( Callicebus nigrifrons) from two different groups (D and R). In over 10 years of behavioural observations of five habituated groups of this species, only low levels of inter-group tolerance have been observed. However, on one day, we encountered the adult male from group D limping (poor hind limb motor coordination) as he travelled alone along the ground. Interestingly, we observed that members of group R did not express any agonistic behaviour towards this neighbouring male and apparently allowed this disabled individual to follow them in the forest for over 5 h. They stayed low in the forest (< 2 m above the ground) and < 10 m horizontally from the individual, and remained in visual contact with him. At the end of the day, this male from group D slept in the sleeping site of group R and was groomed by the adult female of group R. Such tolerance between members of different groups has never been previously observed in this species. Furthermore, group R exposed themselves to increased predation risk by staying close to the ground for protracted periods. The behaviour of group R could be interpreted by as a putative case of empathic responding in this species.